Heat exchangers typically are formed of a plurality of tube and fin assemblies, the ends of which extend through apertures in opposed header plates. A heating or cooling fluid, e.g., oil, air, etc. flows through the tubes. The tube and fin assemblies must be able to withstand system operating pressures without leaking. Elastomeric seals are sometimes used within the apertures in the header plates to seal the tube within the header plate thus forming a tube to header plate joint. Some heat exchanger designs allow the tube and fin assemblies to be removable such that a damaged tube can be replaced without dismantling an entire heat exchanger. According to prior art heat exchangers, the tubes are removed by raising a first end of a tube into the first of two header plates via a first aperture having a cylindrical seal along the entire length of the header plate opening until the second end of the tube clears the second of two header plates. The second end of the tube is swung outwardly to clear the edge of the second header plate and the tube, which is now angled relative to the first header plate, is then pulled from the first header plate, freeing it from the heat exchanger. Prior art aperture and seal designs allow an elastomeric seal to seat within the first header plate in order to seal the tube during heat exchanger operation, and also allow angular movement of a tube for removal and installation from the header plates. See, U.S. Pat. No. 3,391,732, U.S. Pat. No. 4,344,478, U.S. Pat. No. 4,216,824, U.S. Pat. Nos. 4,930,568, 5,433,268 each of which are hereby incorporated by reference in their entireties. However, prior art aperture and seal designs do not optimize the ability of the heat exchangers to withstand high operating pressures while also allowing easy removal and installation of individual heat exchanger tubes. Accordingly, it is an object of the present invention to provide an aperture and seal configuration to improve the ability of a heat exchanger to withstand high operating pressures while also allowing easy removal and installation of individual heat exchanger tubes. It is an additional object of the present invention to provide a heat exchanger header plate which reduces or wholly overcomes some or all of the difficulties inherent in prior known heat exchangers having field removable heat exchanger tubes such as pressure capabilities, seal integrity, and overall heat exchanger life expectancy. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.